is.

to mark it as a distinct word and avoid misreading in handwritten manuscripts.

"The reason for writing I is ... the orthographic habit in the middle ages of using a 'long i' (that is, j or I) whenever the letter was isolated or formed the last letter of a group; the numeral 'one' was written j or I (and three iij, etc.), just as much as the pronoun." [Otto Jespersen, "Growth and Structure of the English Language," p.233]

The form ich or ik, especially before vowels, lingered in northern England until c.1400 and survived in southern dialects until 18c. The dot on the "small" letter -i- began to appear in 11c. L. manuscripts, to distinguish the letter from the stroke of another letter (such as -m- or -n-). Originally a diacritic, it was reduced to a dot with the introduction of Roman type fonts. The basic word for "I" in Japanese is watakushi, but it is not much used. Words that boys usually use are boku (polite) or ore (OH-ray), a rougher word, which can be rude depending on the situation. Girls usually use atashi (a feminine-sounding word) or the neutral watashi, but a tomboy might use boku like boys do.